David Cameron will on Wednesday try to prevent the health service becoming Labour’s election-winning weapon when he guarantees that a majority Conservative government will protect the English NHS budget in real terms from 2015 to 2020.

It is the first time the prime minister has pledged to ringfence the health service from spending cuts for the next parliament, but his promise will still leave the NHS under massive cost pressures due to soaring demand for NHS services largely caused by an ageing society.

Extending a spending pledge that applies in the current parliament, Cameron will say “The next Conservative government will protect the NHS budget and continue to invest more” – and will promise that “the investment will mean more parents have answers to the children’s illnesses and hopefully the cures that go with them”.

In his closing speech to the Conservative conference in Birmingham, the prime minister will also try to broaden the terms of an NHS debate that often seems unwinnable for the Tories by asserting “you can only have a strong NHS if you have a strong economy”.

Cameron’s pledge will be seen as a direct response to Labour’s efforts to raise the political profile of the NHS when Ed Miliband promised last week an additional annual £2.5bn a year “time to care fund” funded by a mansion tax, closing tax loopholes and fees on the tobacco companies.

The value of Cameron’s pledge was not yet quantified, but the coalition’s existing pledge to maintain NHS spending in real terms means that the English health service will have received £12.7bn more in cash terms over the current parliament, meaning a similar sum is likely to apply in the next five years.

Cameron will also reflect on Britain’s frayed social fabric, exposed by the surge in support for the Scottish Nationalists and Ukip, by saying he would like to use a second term in government “to create a Britain that everyone is proud to call home”.

With another conference day preoccupied by rumours of the imminent defection of a third Conservative MP, following the departures of Douglas Carswell and Mark Reckless, Cameron will try to use his speech to argue he can bind some of the wounds created by austerity. He will try to persuade voters that better times are ahead, saying: “If our economic plan for the past four years has been about our country and saving it from economic ruin, the next five years will be about creating a society where hard work is really rewarded.”

The prime minister will add: “I did not come into politics to make the lines on the graph go in the right direction.”

His ambition he will claim “is not a free-for-all, but a chance for all”. The Conservatives regard the pledge to allow NHS spending to rise in line with inflation until 2020 as highly significant since the NHS represents a seventh of public spending and will increase the pressure for £13bn of spending cuts on remaining unprotected departments, such as the Home Office and Business department. Some rightwing Tories such as Liam Fox have called for the protections to be lifted.

Conservative sources said that at this stage no promise had been made to protect the schools budget or the aid programme, the two other budgets that have been protected from real-terms cuts since 2010.

Labour holds a 30-point lead over the Conservatives as the party with the best policies on healthcare, according to polling by IPSOS-MORI published in September. A total of 46% said the Labour party had the best policies, 16% said the Conservatives and 6% the Liberal Democrats. The polling company say this is the lowest score for the Conservative party since September 2004 and the highest for Labour since May 1998.

Although health is traditionally a strong point for Labour – it held a 63% lead in October 1995 – Labour officials are trying to raise its importance with voters over the next few months.

Health analysts, such as the Nuffield Trust, the NHS Confederation and the Kings Fund, have all been calling for greater honesty from all political leaders warning a commitment not to cut NHS spending, or even increase spending by £2.5bn annually does not address the scale of the pressures on the health service.

The Nuffield Trust warned in July the NHS was no longer securing the 4% a year efficiency savings required, so increasing the likelihood of a financial crisis before the election. It warned that by 2021 there would be a £30bn spending gap without vast productivity improvements.

A Kings Fund-commissioned report recommended a single ringfenced budget for health and social care adequately funded by a rise in health and social care spending to 11–12% of GDP by 2025.

In his highly political speech to conference the health secretary, Jeremy Hunt, admitted on Tuesday the NHS had been through the biggest squeeze on finances in its history, and possibly faced a difficult winter ahead, but he blamed Labour for wrecking the economy.

Announcing plans for seven-days-a-week access to GP services by 2020, he said “securing the NHS budget is not about an extra billion here or there”, adding any increase in spending had to be accompanied by reform including greater use of the independent sector, insisting this did not represent privatisation.

Reflecting the anxiety in Tory ranks, he told the shadow health secretary, Andy Burnham, “not to scaremonger about privatisation that is not happening. It nearly cost us Scotland and we won’t let it poison the debate in England.”

He said Labour still tried to trick the public into thinking one party cares for the NHS and the other does not and urged Miliband: “Don’t turn the NHS into a national political football and don’t use the NHS to divide us when its the fabric of the nation.”